The present invention concerns a method of preventing the accumulation of an alkali sulfate which is formed as a by-product in the system in which a method for removal of sulfur dioxide from an exhaust gas of combustion is executed, by which method sulfur dioxide in the exhaust gas is absorbed in an aqueous solution containing an alkali sulfite, and the thus formed acidic alkali sulfite in solution is brought into reaction with calcium carbonate to regenerate the alkali sulfite and to form calcium sulfite; then the calcium sulfite is filtered off, to be removed from the system.
Recently, as a method of removing sulfur dioxide from an exhaust gas containing sulfur dioxide such as exhaust gases of combustion, a method has been brought into operation, in which an aqueous solution containing an alkali sulfite such as sodium sulfite as a main component is utilized as a solution for absorption of sulfur dioxide and then calcium carbonate is added to an aqueous solution containing acidic alkali sulfite formed from the absorbed sulfur dioxide in order to convert the acidic alkali sulfite into calcium sulfite and to remove the absorbed sulfur dioxide in the form of calcium sulfite from the whole system.
Aqueous solution containing an alkali sulfite utilized in the above-mentioned method for removing sulfur dioxide is excellent in the capability to absorb sulfur dioxide and it does not cause the scaling nor clogging in the gas-scrubber in which an exhaust gas is made to contact with the above-mentioned aqueous solution so that the above-mentioned method for removing sulfur dioxide is advantageous for stable operation.
However, because of the conversion of alkali sulfite to alkali sulfate, having no ability to absorb sulfur dioxide, due to gradual oxidation in the step of contact of the aqueous solution of alkali sulfite with the exhaust gas oxygen, the ability of the aqueous alkali sulfite solution as an absorbing solution for sulfur dioxide becomes impaired in the case where absorption of sulfur dioxide is carried out successively.
Hitherto, as a countermeasure of the above-mentioned problem, the following method has been adopted in which (i) a part of the above-mentioned solution for absorption is discharged to the outside of the whole system and alkali sulfite or alkali hydroxide is newly replenished, or (ii) calcium sulfite and sulfuric acid are added to the above-mentioned solution for absorption to form alkali sulfate in the solution and then the sulfate is discharged from the system to the outside in the form of gypsum, the above-mentioned reaction being as follows: EQU M.sub.2 SO.sub.4 +2CaSO.sub.3.1/2H.sub.2 O+H.sub.2 SO.sub.4 +3H.sub.2 O.fwdarw.2MHSO.sub.3 +2CaSO.sub.4.2H.sub.2 O (1)
M denotes an atom of alkaline metal, also hereinafter.
Moreover, as a method for removing the above-mentioned alkali sulfate, (iii) a method is considered in which alkali sulfate is converted into alkali hydroxide by utilizing electrolysis, or (iv) another method is considered in which calcium hydroxide is added to the above-mentioned solution for absorption to convert alkali sulfate into alkali hydroxide and gypsum according to the following reaction (2): EQU M.sub.2 SO.sub.4 +Ca(OH).sub.2 +2H.sub.2 O.revreaction.2MOH+CaSO.sub.4.2H.sub.2 O (2)
However, these methods have their respective disadvantages, that is, the method (i) causes the loss of an effective and expensive alkali source; the method (ii) requires sulfuric acid as a new raw material which is not only expensive but also dangerous in handling; the method (iii) which utilizes electrolysis is susceptible to minute amounts of impurities, necessitating pre-treatment and moreover, in the method (iv), because of the equilibrial nature of the reaction (2) and of the slight solubility of calcium hydroxide, the concentration of regenerable alkali hydroxide is as low as 0.05 mole/liter. Thus, these methods are not satisfactory.
The inventors, as a result of studying the methods of discharging the alkali sulfate formed as a by-product from the whole system have found that in a double decomposition in which acidic alkali sulfite formed from the absorbed sulfur dioxide is converted into calcium sulfite, when granular particles of calcium carbonate of a specified (predetermined) diameter are brought into reaction in two stages with acidic alkali sulfite, gypsum co-separates out with calcium sulfite. In addition, the inventors have found that the above-mentioned alkali sulfate is formed not only in the step of contact of the exhaust gas with the absorbing solution but also is formed in an appreciable amount in the step of filtration of calcium sulfite which is obtained by the conversion of absorbed sulfur dioxide and is removed after filtration.
Moreover, in cases where a vacuum filter is used for filtering calcium sulfite, because of the vigorous gas-liquid mixing given to the filtrate within the filter and the piping from the filter to the gas-liquid separating vessel, the amount of alkali sulfate formed in the step of filtration becomes very large and reach at least more than 10% and up to 50% by weight of the total amount of formation in the whole system.
It is known that carbon dioxide evolves in the step of converting sulfur dioxide absorbed in an aqueous alkali sulfite solution which contains acidic alkali sulfite after absorbing sulfur dioxide into calcium sulfite by the addition of calcium carbonate. It was found by the inventors that when the above-mentioned calcium sulfite is filtered in an atmosphere of the carbon dioxide, the formation of alkali sulfate in the step of filtration is effectively prevented.
Accordingly, the object of the present invention is to furnish a method for preventing the accumulation of alkali sulfate which is produced as a by-product in a system of removing sulfur dioxide in an exhaust gas in the form of calcium sulfite, in which system sulfur dioxide is absorbed by an aqueous alkali sulfite solution as acidic alkali sulfite and calcium carbonate is added to the solution in order to convert the acidic alkali sulfite into calcium sulfite and then calcium sulfite is separated and removed to outside of the system.